Known automatic or automated motor vehicle transmissions normally feature a parking lock, by way of which an output shaft of the transmission can be mechanically fixed. In addition to a purely mechanical system for actuating this kind of parking lock such that a mechanical functional connection between a selector lever operable by the driver in the interior of the motor vehicle and the parking lock in the transmission, there are also known electromechanical and electrohydraulic systems for actuating this type of parking lock—for example from DE 41 27 991 C2—in which the parking lock in the transmission is connected via an electric functional connection with a selector device operable by the driver in the interior of the motor vehicle, and in which the parking lock can be electromechanically or electrohydraulically actuated by way of a parking-brake function implemented in an electronic control device of the transmission, depending on a drive position selected on the selector device and depending on other operating parameters of the motor vehicle.
In addition, there are various known methods of automatically engaging this kind of parking lock. U.S. Pat. No. 3,937,105 proposes automatically engaging the parking lock of the transmission independently of the driving position selected by the driver if an ignition circuit of the motor vehicle is interrupted and the motor vehicle is still coasting at a speed that is lower than a defined low threshold value.
A method is known from U.S. Pat. No. 4,892,014 in which the parking lock of the transmission is engaged, on the one hand automatically by way of an electric motor when an ignition circuit of the motor vehicle is interrupted and at the same time the vehicle speed is lower than a defined low threshold value, and on the other hand, when the driver's door of the motor vehicle is open and at the same time the driver's seat of the motor vehicle is unloaded, and at the same time the vehicle speed is lower than the cited low threshold value. In both cases, it is provided that when the parking lock is automatically engaged, a parking brake of the motor vehicle is actuated as well. In U.S. Pat. No. 4,892,014, it is also proposed that when the ignition circuit is closed when the drive motor of the motor vehicle is not running, the transmission automatically switches to its neutral position, without a driver request being necessary.
And finally, from EP 0 814 287 B1, a function for automatically engaging the parking lock of an automatic transmission is known, with which the parking lock in the automatic transmission is automatically engaged by the transmission control device when the speed of the motor vehicle is zero and at the same time an ignition circuit is interrupted, and at the same time, as an additional condition, either a predetermined time interval has passed since the interruption of the ignition circuit or a vehicle door is opened, but no later than—and then as the only condition—when the ignition key has been removed from the ignition switch. In order to achieve better vehicle availability, it is a special feature in all three cases that the engagement of the parking lock is prevented when the driver, immediately before switching off the motor or within a predetermined time interval after switching off the motor, has selected the neutral drive position with the selector lever.
The object of the invention is to devise a function for automatically engaging the parking lock of an automatic or automated transmission that is an alternative to the state of the art, which on the one hand sufficiently protects the vehicle against start-up or rolling not intended by the driver, and on the other hand, has the least possible impairment of vehicle drivability, while avoiding, as far as possible, a change in the operating state of the transmission or the vehicle that is surprising to the driver.